Problem with factoring out common code in Rust

rust_factor_out_tuple_issue.rs

// Read from DB, write data to CSV
extern crate csv;
extern crate oracle;
use std::error::Error;
use std::env;

use oracle::Connection;
use oracle::sql_type::FromSql;
use oracle::row
use serde::ser::{ Serialize };

// FIXME: T should be a tuple of items implementing `FromSql + Serialize` due to `conn.query_as::<T>`
// SOLUTION: `fn sql_to_csv<T: RowValue + Serialize>` + `use oracle::RowValue;`
fn sql_to_csv<T: FromSql + Serialize>(psw: &str, sql: &str, colnames: &[&str], filename: &str) -> Result<(), Box<dyn Error>> {
    let conn = Connection::connect("admin", psw, "//localhost:1541/MYDB")?;
    let rows = conn.query_as::<T>(sql, &[])?;

    let mut wtr = csv::Writer::from_path(filename)?;
    wtr.write_record(colnames)?;

    for row_result in rows {
        wtr.serialize(row_result?)?;
    }

    wtr.flush()?;
    println!("Written {}", filename);
    Ok(())
}

fn main() -> Result<(), Box<dyn Error>> {
    println!("Hello, world!");
    let psw = env::var("DB_PSW").expect("Env var DB_PSW is required");

    // Fails with "the trait `FromSql` is not implemented for `(std::string::String,)`"
    sql_to_csv::<(String,)>(
        psw.as_ref(),
        "select ORGANIZATION_NUMBER from organization",
        &["ORGANIZATION_NUMBER"],
        "organization.csv"
    )?;

    // Fails with "the trait `FromSql` is not implemented for `(std::string::String,std::string::String,std::string::String)`"
    sql_to_csv::<(String, String, String)>(
        psw.as_ref(),
        "select SUBSCRIBER_ID, PROFILE_ID, ORGANIZATION_NUMBER \
        from SUBSCRIBER",
        &["SUBSCRIBER_ID", "PROFILE_ID", "ORGANIZATION_NUMBER"],
        "subscriber.csv"
    )?;

    Ok(())
}